Mesophase pitch-based carbon fibers, having a highly ordered morphology, are well known in the art. That is, when heated, pitch materials form an isotropic mass. As heating continues, spherical bodies begin to form. The spherical bodies are of an anisotropic liquid crystalline nature as viewed under polarized light. These spheres continue to grow and coalesce until a dense continuous anisotropic phase forms, which phase has been termed the “mesophase.” Thus, the mesophase is the intermediate phase or liquid crystalline region between the isotropic pitch and the semi-coke obtainable at higher temperatures. Mesophase pitch is extruded into fibers, oxidatively stabilized, and carbonized at high temperatures to form mesophase pitch-based carbon fibers.
As compared to PAN-based carbon fibers, these mesophase pitch-based carbon fibers have a relatively high tensile modulus but a relatively low tensile strength and a relatively low compressive strength. That is, the highly graphitic structure of these polymers is responsible for their high modulus and high thermal conductivity, but also for their low strength. The structured, crystalline morphology that provides for the high tensile modulus also allows for the brittle failure of the fibers caused by the propagation of even minute flaws. Thus, mesophase pitch-based carbon fibers are known to suffer from an imbalance of physical properties.
In order to reduce the tendency of mesophase pitch-based carbon fibers to fail through flaw propagation as described above, a variety of prior art patents have provided methods for producing such fibers with a less ordered, more random microstructure. To date, these efforts have focused on the method by which the fiber is extruded, including modification of the geometry of the die through which the fiber is extruded, rather than on composition modification.
Carbon nanotubes are very small carbon-based tubes having a hollow core having an average diameter of 5 to 10 nanometers. About the core is either a single wall of carbon or several walls. The walls resemble a hexagonal lattice of carbon rolled into a cylinder. Multi-wall carbon nanotubes average between about 30 and about 50 concentric, cylindrical walls. Carbon nanotubes are extremely stiff, having an average modulus of from about 400 to about 4,000 GPa.
Carbon nanotubes have been incorporated into isotropic pitch and made into isotropic pitch-based carbon fibers having carbon nanotubes ranging in amounts 1 percent by weight to 5 percent by weight and have been found to increase elastic modulus and conductivity as well as tensile strength.